Set a different default PPA depending on network

Summary:
Setting --network does not work out of the box as it will
try to upload to the ABC PPA. This fixes that by providing a sane default
for the BCHN PPA when BCHN is selected as a network.

Test Plan:
Verify .dput.cf is created as expected:

rm ~/.dput.cf
./debian-packages.sh jasonbcox
cat ~/.dput.cf

Test various non-default cases:

./debian-packages.sh -n ABC jasonbcox
./debian-packages.sh -n BCHN jasonbcox
./debian-packages.sh -p doesnotexist jasonbcox

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8456